The present invention relates to a device for supplying voltage to an electrical machine, in particular a paper-processing machine, having a plurality of electric drives, electric drives of the same supply voltage being combined into one voltage level.
Printing presses and folding machines have a plurality of electric drives. In this context, the power of the electric drives is of different orders of magnitude, depending on the task an electric drive has. So, first of all, there are the main drive and larger auxiliary drives of a printing press, which have powers in the range of 1-100 KW. Below that, in terms of power, come the smaller auxiliary drives and the drives for actuating the stacks at the feeder and the delivery, having powers of several hundred watts. A further group is constituted by the small drives with a power of several watts and the supply of the electronics. Since the mentioned drives work in different power classes, they have different power supplies as well. Therefore, the drives of one power category were combined into a voltage level, and this voltage level was connected to the usually existing three-phase system. This results in that the main and larger auxiliary drives are supplied with a relatively high voltage which is produced by transforming the mains voltage, that the smaller auxiliary drives are supplied with a medium voltage, and that the small drives and the electronics are supplied with a low voltage. In this context, each of these voltage levels is provided with a separate mains connection, i.e., in each case, one voltage transformer is required which converts the mains voltage to the required voltage level. Since the main and auxiliary drives of a printing press or folding machine can also operate regeneratively, it must be made possible for surplus electric energy either to be fed back from the main and auxiliary drives of a printing press or folding machine into the mains supply or to be converted to heat using electrical resistors (choppers) to be switched in. This results in that each voltage level in which drives can also be operated regeneratively must be provided with a chopper for removing peak loads or that, in each case, a so-called “feedback unit” must exist between a voltage level and the mains supply to allow electric energy to flow back from the voltage level to the mains supply.